CN110949015B

CN110949015B - Two-stage density compensation method

Info

Publication number: CN110949015B
Application number: CN201910783175.7A
Authority: CN
Inventors: A·豪克; I·特拉莎娜斯; B·施特里泽尔; F·克洛普纳
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 2018-09-26
Filing date: 2019-08-23
Publication date: 2021-12-14
Anticipated expiration: 2039-08-23
Also published as: JP2020049943A; CN110949015A; US20200094575A1; DE102019212739A1

Abstract

The invention relates to a method for computer-aided compensation of position-dependent density fluctuations in the printing nozzles of an inkjet print head of an inkjet printer (3), comprising the steps of: pre-compensating, by means of a pre-press stage computer (1), all color separations of the printed image (8) during the rasterization process in the pre-press stage on the basis of a pre-specified density compensation profile; all color separations of the pre-compensated printed image (9) are compensated online during the production of the printed image (10,11) in the main printing by the control computer of the inkjet printer (3) on the basis of the newly calculated density compensation contour curve.

Description

Two-stage density compensation method

Technical Field

The invention relates to a method for compensating for density inhomogeneities transverse to the printing direction in an inkjet printer.

The technical field to which the invention belongs is ink jet printing.

Background

One important quality indicator in printing is the realization of a clear and well-defined mapping (Abbildung) of image content/material. Here, the mapping on all locations within the printable layout should produce the same result, which requires uniformity in the printing in terms of location. The difficulties with digital printing are: this required uniformity is ensured transversely to the printing direction (X-rail). Generally, inkjet print heads have some scatter (Streuungen) for the jetting of a single nozzle. The reason for this is that: scattering in production, scattering in the ink supply of the printing nozzles, scattering in the mechanical scattering of the piezoelectric actuators, etc. All this leads to: when multiple printing nozzles are uniformly loaded, non-uniformity occurs through the density (or coloration) of the print head.

This problem is solved by means of a density compensation method. This density non-uniformity, with all printing nozzles uniformly controlled, is achieved by means of a test shape (testform) for a specific amount of dot coverage

And (5) collecting. Depending on the course transverse to the printing direction, a compensation contour curve (kompensationprofile) is obtained and maintained accordingly during the printing process, so that ultimately a uniform mapping is achieved in this respect.

In order to solve the object of the invention, two different method approaches are disclosed and can also be used in existing printing systems.

1. Compensation is carried out in a site-dependent calibration mode during the rasterization process:

here, these grids include 4096 different pattern patterns (Muster), wherein the pattern numbered 0 represents a pure white surface and the pattern numbered 4095 represents a pure black surface. The hierarchy between these two values is tone value stair-stepping (tonwerttrepappe) and grows monotonically. If no calibration is used, 256 input values are mapped to 4096 pattern styles as follows:

gray scale value 0 → pattern 0,

gray value 1 → the pattern 16,

……，

grayscale value 255 → pattern 4095.

By changing the mapping rules, both the alignment (making the steps no longer equidistant) and the ink limit (making that pattern for gray value 255 less than 4095) can be adjusted. This approach has long been established and used in various grid processors.

In order to achieve density compensation based on this approach, the position within the row line (Zeile) is then taken into account for the mapping rule. For this purpose, a Lookup Table (LUT) is calculated from the density profile (Dichteprofil) and from the calibration method, which contains the number of the pattern to be used for the combination of position in the row line and gray value. The pattern is then used in the rasterization process.

The method has the advantages that:

the mapping properties of the grid are maintained in this way, in particular grain, noise, orientation, etc. are not negatively affected. The knowledge about the dot coverage required for the above calculation is known from the image content with high resolution, and, unlike method 2, no additional determination is required.

The disadvantages of this approach are:

this compensation is performed during the rasterization process, namely: if the compensation profile changes, a new rasterization process is required. This is relatively time consuming and can only be done online under certain conditions. If the printed image is displaced transversely to the printing direction and thus relative to the printing unit, a re-rasterization is also required. On the other hand, after the rasterization, it is no longer possible to effect a change in the lateral registration or a change in the position of the sheet in the machine (or the position of the web transverse to the printing direction), since the local compensation intensity is shifted together with the image relative to the print head and the assignment is no longer correct.

2. Compensation is performed on the rasterized image in a single color separation (online compensation mode):

in this approach, density compensation is based on the rasterized color separation. For the part to be compensated in the image, the current dot coverage area is firstly obtained based on the grid. This process is not carried out with high resolution on the image information, but on the rasterized image by means of a matrix (for example a window of 3 × 9 pixels), i.e. by averaging the number of raster points. The associated compensation contour curve (or local value) is then calculated by interpolation using the dot coverage area, and the pixel values, distribution and intensity are adapted accordingly, so that an image setpoint value is generated. This process is carried out in an online manner (online) during the printing process by means of the control computer of the inkjet printer. The raising or lowering of the image value is achieved by adding (or deleting) dots (Dot) or by raising or lowering the Dot size produced by the volume of the ejected ink drop.

The method has the advantages that:

this compensation process is performed online and very quickly becomes feasible on the fly, without interrupting the printing process. For example, when registration changes, the shifting of the image relative to the print head is still feasible without problems.

The disadvantages of this approach are:

the local dot coverage is sought by a sliding window, which may lead to formations (artfakte) at the edges of the surface elements or in the case of linear elements in combination with this method.

The rasterized image is intervened by using corrective measures so that the grid changes. This is perceived as disturbing (in the form of structures, particles, noise or other formations), in particular in the case of strong corrections. Such grid algorithms are typically optimized for image properties such as uniformity, resolution, smoothness, robustness, etc. Intervening grid configurations may result in poor quality.

Another problem with inkjet printing is the so-called white line, i.e.: the formation of stripes in the printing direction is caused by an insufficient functioning or even a complete stoppage of the individual printing nozzles. For these false images, there is an inherent method means for compensation. The method can be carried out in the rasterized image during the printing process at the same time as the density difference compensation. In the case of white lines to be compensated which occur frequently in places, density compensation is particularly difficult to carry out here, and visible inhomogeneities often remain. Thereby, the white line compensation also affects the compensation of the density fluctuation. For the method for compensating for locally occurring density fluctuations, it is therefore also necessary to take into account the white lines that occur or the measures for compensating for them accordingly.

Disclosure of Invention

The object of the present invention is therefore to provide a method for compensating for density fluctuations in an inkjet printing head of an inkjet printer, which combines the advantages of the various methods known for this purpose without the disadvantages of the corresponding known methods.

This object is achieved by a method for the computer-supported compensation of position-dependent density fluctuations in the printing nozzles of an inkjet print head of an inkjet printer, comprising the following steps: pre-compensating, by a pre-press stage computer, all color separations of the printed image during the rasterization process in a pre-press stage based on a pre-given density compensation profile curve; the density compensation contour curve obtained by the control computer of the inkjet printer on the basis of the new calculation is used to compensate all color separations online during the execution of the printed image in the main printing. In order to correspondingly take full advantage of the advantages of both methods, both methods are combined and adapted to each other. The adaptation mode is as follows: a first known method of compensation during the rasterization process by means of adaptive calibration is carried out as a first step for the pre-compensation type. This pre-compensation of the position-dependent density fluctuations occurring in the printing nozzles of the inkjet print head already makes it possible to exclude most deviations in the printing behavior of the printing nozzles. Then, if other problems arise during the printing process with regard to fluctuations in the density of the printing nozzles, these problems are solved within the scope of an online compensation using a second method of the prior art, in which the density fluctuations that arise are compensated for by evaluating the local dot coverage area and adapting the dots (or dot size) generated for the ink drops used during printing on the basis thereof. For this purpose, test prints must of course be carried out regularly in order to record the density fluctuations that newly arise therefrom. Since most of the density fluctuations are already excluded in the context of the pre-compensation, the adaptation to be carried out in the context of the online compensation is significantly less in this case than in the case of the online compensation method alone. Thus, the disadvantages of such an online compensation method are avoided in the final effect, namely: so that the image formation resulting from the intervention of the existing grid of the grid image already present at the moment of printing can be reduced as much as possible. The disadvantages of the first method of the prior art disclosed in this respect are also avoided by rasterizing the respective print image to be generated only once and then no longer by means of the respective adapted calibration method, namely: the costly rasterization process must be repeated in order to be able to compensate for the density fluctuations that occur during the printing process. Thus, combining the two methods according to the invention enables an effective compensation of the position-dependent density fluctuations occurring in the printing nozzles, which compensation takes advantage of the two methods but avoids the associated disadvantages.

Advantageous and therefore preferred developments of the method result from the preferred embodiments and the description with the figures.

In this case, a preferred development of the method according to the invention consists in that the predefined density compensation profile for the precompensation method is created by means of test measurements and/or on the basis of density inhomogeneities filed by the print head manufacturer, while the newly calculated density compensation profile for the online compensation method is calculated by the control computer of the inkjet printer on the basis of a comparison between the current setpoint values and the measured actual values of the dot coverage for all color separations. Both of these compensation methods require corresponding density compensation profiles. This compensation profile gives: which printing nozzles of an inkjet printing head of an inkjet printer have to print more or less strongly to which extent in order to equalize the density fluctuations inherent to the individual printing nozzles themselves. The compensation contour curve is therefore always present in the form of a location-dependent function which determines the amplitude (i.e. the printing intensity) of each individual printing nozzle. Since the precompensation method is implemented as a first method step in the prepress phase by adapting the calibration of the rasterization process, the compensation profile or profiles required for this purpose need already be supplied to the computer at this point in time. This can be created by test measurements prior to the printing process, wherein the density fluctuations of the individual printing nozzles must first be detected initially and then a compensation contour curve corresponding to the detected density fluctuations is created accordingly. Alternatively, a factory (ab Werk) compensation profile curve provided by the manufacturer of the inkjet print head may be used. The choice of which mode depends on the conditions to which the printing process is adapted, the inkjet printer used and the experience of the user. It is also possible to start with compensation profile curves of the print head manufacturer and then update these compensation profile curves by other test measurements. In addition to or instead of the test measurement, the second substep of the method with respect to the online compensation, which is carried out during the execution of the printing process, can also be used to measure the resulting dot coverage area using the printed product itself, which is realized during the printing process, as a starting point. Then, similar to test measurements for the pre-compensation mode method, the current compensation profile or profiles can be calculated based on these values. This is of course necessary for all existing print heads for all color separations (i.e., ink mechanisms).

In this case, a further preferred development of the method according to the invention consists in that, in the event of a register change transverse to the printing direction, the density compensation contour curve calculated by the control computer of the inkjet printer is corrected by the control computer in that the register change difference (Delta) between the predetermined original density compensation contour curve and the predetermined density compensation contour curve shifted as a result of the register change is subtracted from the calculated density compensation contour curve. If register changes occur during the execution of the printing process, this must of course be incorporated into the compensated profile curve calculated during the on-line compensation. Otherwise, the location-dependent compensation intensity for each individual printing nozzle based on the compensation contour curve is no longer used for the printing nozzle on which it is based, but for the adjacent printing nozzle. To avoid this, the registration variation delta is subtracted accordingly.

In this case, a further preferred development of the method according to the invention consists in that the precompensation method first corrects the density inhomogeneities associated with the print head, while the in-line compensation method preferably excludes the influence of the printing substrate. Depending on the state of the printing process, for the compensation process in the form of two steps, in each compensation step, the interference effects prevailing at this point in time of the printing process are also preferably corrected. For the pre-compensation method, this is more precisely for print head-related inhomogeneities (i.e. density fluctuations), as it is also mainly disclosed in the print head manufacturer's data (as long as the print head manufacturer does not at the same time provide a compensation profile curve derived on the basis thereof). Once these print head related non-uniformities are first corrected by the pre-compensation method, they need only be slightly altered during online compensation. In contrast, in the latter (online compensation), the substrate effects are more targeted.

In this case, a further preferred development of the method according to the invention consists in the control computer of the inkjet printer taking into account the influence of the different grids in the pre-compensation during the online compensation. That is, the two compensation steps in the method according to the invention are not completely independent of each other. The influence of different grids in the precompensation for the elimination of the density fluctuations (or inhomogeneities) known here is of course also relevant for the type of grid used in the rasterization process. Since these grids are then used in each case when the actual printing process is carried out, since the rasterized printing image is printed by an inkjet printer, it is also expedient to consider them as influencing factors for the online compensation. However, this does not mean that re-rasterization is required or performed during online compensation.

In this case, a further preferred refinement of the method according to the invention consists in assigning, during the pre-compensation, within the scope of the calibration raster processor, a specific raster pattern to the specific gray values of the color separations to be rasterized of the print image by means of a look-up table which contains, as additional variables, the position of each printing nozzle of the printing head of the inkjet printer, for each position of the printing nozzle in the look-up table, the entire set of gray values with the assigned and adapted raster pattern is respectively entered by the prepress stage computer, these raster pattern patterns are used by the prepress stage computer for rasterizing the print image, and the rasterized image is printed on the inkjet printer. That is, the pre-compensation method uses a known method for compensation during rasterization. This is based on the fact that it is anyway necessary to calibrate the grids used for the rasterization process. Instead of using a look-up table in the rasterization domain, which as hitherto has been assigned a specific raster pattern for each gray value, it is also possible to additionally provide the rasterization process with the position of each printing nozzle. The calibration process, i.e. the assignment between a specific gray value and a specific grid pattern, can thus be correlated to the performance of each printing nozzle. For example, if a printing nozzle is printed slightly less than expected, and thus requires enhanced ink injection in the context of density compensation, a corresponding grid with a corresponding increased ink injection can be assigned to the printing nozzle in order to equalize the effect of density fluctuations. That is, the calibration that would otherwise be required is used simultaneously to equalize the local density fluctuations together. For this purpose, it is of course necessary to know the density fluctuations of these individual printing nozzles before calibration. For this purpose, these density fluctuations must be measured accordingly in advance. However, since density compensation is required continuously, the current values of the density fluctuations for these individual printing nozzles always need to be determined regularly. In addition to this, the position of each printing nozzle is introduced as a possible variable into the look-up table, which expands the look-up table in the form of a matrix. That is, instead of only one look-up table assigning a specific grid to each gray value, there are n look-up tables, where n depends on the number of used positions of each printing nozzle.

In this case, a further preferred refinement of the method according to the invention consists in that the adapted assignment between the grid pattern and the gray-scale values for each position of the printing nozzles in the look-up table is dependent on the density fluctuations of the individual printing nozzles, respectively. This adapted distribution relationship ensures that the locally occurring density fluctuations that each printing nozzle has can already be effectively compensated for by the calibration of the grid. For example, if too small an ink discharge quantity of the respective printing nozzle leads to too small a color value in the printed image, this density fluctuation can be compensated for by an adapted assignment between the grid and the desired gray value. Since the density fluctuations (in this case the occurring too small color values) are known during calibration, the corresponding gray values for the printing nozzle (i.e. the position) are used with higher grid values than in the conventional case. This correspondingly compensates for the functional deficiency of the printing nozzles.

In this case, a further preferred refinement of the method according to the invention consists in implementing a maximum ink limit (maximum tintenbergenzong) by assigning a lower grid pattern to higher gray values (compared to the case of an equidistant normal distribution) by means of a prepress stage computer. Ink-jet printing is known for the so-called maximum ink limit, since (unlike offset printing) ink-jet printing is essentially unable to register separations in a superimposed manner. Too much ink on a specific part of the substrate can lead to negative effects (for example with regard to drying performance or the state of the substrate). The maximum ink limit can be performed very simply in the calibration domain. Instead of correspondingly assigning a higher gray value (for example the highest gray value 255) to a higher grid pattern with a very high ink quantity (for example the maximum value 4095), intervention is carried out here. A value of 4095 may result in a high amount of ink such that the maximum ink limit may have been exceeded. Thus, in the context of calibration, higher gray values should not be assigned such higher grid pattern patterns (which should not be equally spaced as in the case of converting 8-byte gray values into 12-byte grid pattern patterns), but correspondingly lower grid pattern patterns should be used. For example, for a gray value 255, a grid pattern with the number 3172 is also completely sufficient. It is important here that the spacing between the individual raster pattern patterns, which are respectively assigned to the raised gray values, is reduced as required, so that the color plausibility (i.e. the target color values to be achieved for the respective print job) continues to exist.

In this case, a further preferred development of the method according to the invention consists in that during the online compensation, the control computer determines the current dot coverage area for the region to be compensated in the print image on the basis of the grid by means of a matrix, calculates the associated density compensation contour curve, and adapts the print image data in such a way that a desired value for the dot coverage area is achieved. The advantages which have been shown are: for the part to be compensated in the image, firstly, the current dot coverage area is obtained based on the grid. In this case, this process is not carried out at high resolution on the image information, but is calculated on the rasterized image by means of a matrix (for example a window of 3 × 9 pixels), i.e. by averaging the number of raster points. The associated compensation contour curve (or local value) is then calculated by interpolation using the dot coverage area, and the pixel values, distribution and intensity are adapted in this way, so that the desired image value is generated.

In this case, a further preferred development of the method according to the invention consists in adapting the print image data by the control computer by adding (or deleting) pixels or by controlling an increase or decrease in the volume of the ejected ink drops in the pixel size. This is a preferred way to adapt the increase or decrease of the image value of the printed image data as necessary.

In this case, a further preferred development of the method according to the invention consists in that systematic density fluctuations occurring during the online compensation are excluded from the pre-compensation in subsequent print jobs. This of course only relates to density fluctuations which are not directly related to the print job. Furthermore, a data connection between the control computer of the ink jet printer, which performs the online compensation, and the prepress stage computer is necessary in order to provide the prepress stage computer with the data required for the precompensation.

Drawings

Such a method and structurally and/or functionally advantageous refinements thereof are further described below on the basis of at least one preferred embodiment with reference to the drawing. In the drawings, elements corresponding to each other are denoted by the same reference numerals, respectively.

The figures show:

FIG. 1: structural examples of inkjet printer systems;

FIG. 2: an example of a rasterization process with calibration and density compensation;

FIG. 3: exemplary progression of the method according to the invention.

Detailed Description

The method according to the invention is applied in an inkjet printer 3 located in a specific workflow system. Such a workflow system is exemplarily shown in fig. 1. Such a workflow system runs on one or more prepress stage computers 1, by means of which prepress stage computers 1 the respective print jobs 5 are processed. In this case, the print job 5 to be printed on the inkjet printer 3 is rasterized by the raster image processor 2, and the rasterized print image 4 is transferred from there to the inkjet printer 3 for the corresponding main printing. The calibration according to the invention is carried out between the gray values of the individual color separations of the prepress image and the corresponding grid pattern 7 by means of a grid image processor 2, which grid image processor 2 likewise runs on a computer 1, which computer 1 can be coordinated with the prepress stage computer of the workflow system, producing a rasterized printing image 4 on these grid pattern patterns 7. According to the invention, local density fluctuations 8 occurring in the inkjet printer 3 are also compensated in the raster image processor 2.

For this purpose, fig. 2 shows how this calibration method for compensating density fluctuations can be adapted accordingly. Here, there is no longer a single look-up table as such: in this single look-up table, the gray values 0 to 255 are in the correspondingly adaptively determined grid pattern 7; instead, the position of the individual printing nozzles is taken into account in addition to the gray values in the look-up table on the basis of the created density profile which has the density fluctuations of the inkjet printer 3. For example, for nozzle position X, the sets of gray scale values 0 to 255 are assigned to the corresponding sets of grid pattern patterns 0 to 4095. For the next printing nozzle X +1, another set of gray values of 0 to 255 is assigned to the corresponding grid pattern 7. For all printing nozzles participating in the print job 5 and which are then to be compensated for density fluctuations, a corresponding set of value pairs is created: i.e. the grey values and the grid pattern 7.

In a particular embodiment variant, such a look-up table 6 can be created even without data of the current print job 5: the look-up table 6 is developed to contain the position of each printing nozzle. For the density fluctuation compensation according to the invention, all that is necessary is: a density profile is created that contains local density fluctuations of the inkjet printer 3.

If the raster image processor 2 is calibrated according to the invention, each print job 5 can be rasterized by means of the look-up table 6 thus created, which look-up table 6 takes into account the position of the individual printing nozzles. In this way, the rasterized print image 4 thus created already contains the machine-specific density fluctuation compensation of the inkjet printer 3 in question. In this way, a correspondingly rasterized print image 4 can be printed with density compensation already being achieved.

In order to avoid the above-mentioned disadvantages of the two known density compensation methods and to use their advantages, it is proposed to combine the above-mentioned methods. Fig. 3 schematically shows the progress of the method according to the invention.

The step 1 comprises the following steps: the color separation of the print image 8 in the current print job is pre-compensated by means of the computer 1 during the rasterization on the basis of such compensation contour curves, or also on the basis of density inhomogeneities which are known for the print heads and are recorded by the print head manufacturer; and determining the settings of the piezoelectric voltages for the individual print heads in the calibration domain. This step can be performed during the rasterization with the aid of fixed profile curves and settings. The frequency of variation of these compensation profiles can be kept low. Thus, after this step, a pre-compensated, rasterized print image 9 is obtained.

In step 2, the remaining and thus minor corrections are compensated for by an online compensation method. This is usually done by the control computer of the ink jet printer 3. By this combination, the final result of the printed image 10, which is additionally compensated online, is independent of position. Here, the variation is covered by step 2 and the intervention on the grid is relatively small, since the remaining residual deviations are small. Thus, formation (e.g. in case of a major intervention) can be avoided. Since the entire compensation method is composed of two partial steps, two contour curves must also be determined (or determined). If a register change now occurs transversely to the printing direction, the first contour curve is also shifted together with the printed image 10 relative to the printing unit, with the result that the contour curve for the online compensation must then be corrected accordingly. The new second contour curve then follows from:

new contour curve 2 is original contour curve 2-difference shifted from contour curve 1 to contour curve 1

Thus, by applying the second profile curve, the result is a printed image 11 that has been compensated online, with the registration variations removed.

The determination of the density profile for calculating the compensation profile 2 must be carried out logically with a precompensation with the profile 1.

In a further preferred embodiment, it is also possible to design such that the precompensation method corrects for print head-related inhomogeneities and such substrate influences are integrated into an online method. The method can further be designed such that the influence of different grids is integrated into the online method as long as different rasterization processes are employed.

By these measures, the proportion of the inhomogeneities precompensated by means of the first density compensation method can be reduced to a proportion of the total error of, for example, 80%. Thereby enabling the basic advantages to be maintained.

List of reference numerals

1 prepress stage computer

2 Raster Image Processor (RIP)

3 ink-jet printer

4 rasterized print image

5 print Job

6 position-dependent, calibrated look-up table (LuT) matrix

7 grid pattern

8 print image of current print job

9 pre-compensated printed image

10 printed image compensated online

11 printed image with on-line compensation, with registration variation removed

Claims

1. A method for computer-aided compensation of position-dependent density fluctuations in the printing nozzles of an inkjet print head of an inkjet printer (3), comprising the steps of:

-pre-compensating, by a pre-press stage computer (1), all color separations of the printed image (8) during the rasterization process in the pre-press stage based on a pre-given density compensation profile curve, wherein the pre-compensation first corrects for print head related density non-uniformities;

-performing an online compensation for all color separations of the now pre-compensated print image (9) during the generation of the print image (10,11) in the main printing by means of a control computer of the inkjet printer (3) on the basis of the newly calculated density compensation contour curve, wherein the online compensation excludes the influence of the printing substrate,

wherein systematic density fluctuations occurring during the online compensation are excluded from the pre-compensation in subsequent print jobs.

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the pre-specified density compensation contour curve for the pre-compensation is created by means of test measurements and/or on the basis of density inhomogeneities filed by the print head manufacturer, while the newly calculated density compensation contour curve for the online compensation is calculated by the control computer of the inkjet printer (3) on the basis of a comparison between the current setpoint values and the measured actual values of the dot coverage areas of all color separations of the pre-compensated print image (9).

3. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

in the case of a register change transverse to the printing direction, a density compensation contour curve calculated by a control computer of the inkjet printer (3) is corrected by the control computer in such a way that the following register change difference is subtracted from the calculated density compensation contour curve: the register change difference is a register change difference between the original predefined density compensation profile and the predefined density compensation profile shifted due to the register change.

4. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the control computer of the inkjet printer (3) takes into account the influence of the different grids in the pre-compensation in the case of the online compensation.

5. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

during the pre-compensation, a specific gray value of a color separation to be rasterized of the print image (8) is assigned a specific raster pattern (7) by means of a look-up table (6) within the scope of a calibration of the raster processor (2),

the look-up table (6) contains, as additional variables, the position of each printing nozzle of the print head of the inkjet printer (3),

for each position of the printing nozzle in the look-up table, recording by the prepress stage computer (1) a complete set of gray values together with the associated and adapted grid pattern (7),

the grid pattern (7) is used by the prepress stage computer (1) for rasterizing the printed image (8), and

printing the rasterized, pre-compensated print image (9) on the inkjet printer (3).

6. The method of claim 5, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

for each position of the printing nozzles in the look-up table (6), the adapted assignment between the grid pattern (7) and the gray value is in each case related to the density fluctuation of the respective printing nozzle.

7. The method of claim 5, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

maximum ink limiting is performed by assigning a lower grid pattern (7) to higher gray values by the prepress stage computer (1) than would be the case if an equidistant normal distribution were followed.

8. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

during the online compensation, the control computer determines the current dot coverage area for the region to be compensated in the print image on the basis of the grid by means of a matrix, calculates the associated density compensation contour curve, and adapts the print image data to a target value for the dot coverage area.

9. The method of claim 8, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

adapting the print image data by the control computer in a manner that: pixels are added or deleted or pixel size is increased or decreased by controlling the volume of ink drops ejected.